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Title:The interaction of dietary phosphorus and exogenous phytase concentrations on nutrient metabolism of beef cattle
Author(s):Long, Chloe Jay
Advisor(s):Shike, Daniel W
Contributor(s):Stein, Hans H; Felix, Tara L
Department / Program:Animal Sciences
Discipline:Animal Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):beef cattle
Abstract:Phytate, the salt of phytic acid, is the main form of phosphorus (P) in cereal grains and by-product feeds and P must be cleaved from phytate to be absorbed. The P associated with phytate can make up to 60 to 80% of total P in cereal grains (Eeckhout and De Paepe, 1994; Viveros et al., 2000). Thus, beef cattle are fed dietary inclusions of P above their requirement to ensure adequate amounts are absorbed. As P intake increases above the requirement of the animal, endogenous fecal P and feed P excretion may also increase (Braithwaite et al., 1985; Scott et al., 1985; Ternouth, 1989), contributing to P pollution of surface water (U.S. EPA, 1996). Reducing P excreted in manure would help mitigate the impact beef cattle have on eutrophication of fresh waters. One way to reduce excreted P is by increasing the availability of P to the animal, thereby increasing absorption. A popular feeding strategy to increase P availability for nonruminants is the dietary addition of phytase, an enzyme that releases P bound to phytate, making it more available for absorption (Lei et al., 1993; Ravindran et al., 2000; Jongbloed et al., 2004). The microbial population in the rumen produces phytases; however, passage rate, Ca-complexes formed with phytate, and processing of dietary ingredients can reduce ruminal phytase activity (Konishi et al., 1999; Park et al., 1999; Kincaid et al., 2005). Limited data exists on the addition of supplemental, exogenous phytase to beef cattle diets to improve P availability to reduce the need for supplemental P. Objectives were to determine the interactions of phytase inclusion and dietary P concentration on metabolism and digestibility of beef cattle fed starch-based diets. Six ruminally-fistulated steers (initial BW = 750 ± 61 kg) were fed in a 6 × 6 Latin square design with a 3 × 2 factorial arrangement of treatments: 1) 0 FTU phytase, 0.10% dietary P, 2) 500 FTU phytase, 0.10% dietary P, 3) 2,000 FTU phytase, 0.10% dietary P, 4) 0 FTU phytase, 0.30% dietary P, 5) 500 FTU phytase, 0.30% dietary P, 6) 2,000 FTU phytase, 0.30% dietary P. Where one FTU is defined as the amount of phytase that liberates 1 μmol of inorganic phosphate per minute from 0.0051 M Na-phytate solution at a pH of 5.5 and 37°C (Engelen et al., 1994). There were no main effects of phytase inclusion on any parameter measured. However, dietary concentrations of P affected (P < 0.01) DMI, total fecal output, and apparent DM digestibility. Steers fed the 0.10% P diet, consumed less DM and excreted less feces, but had increased apparent DM digestibility compared with steers fed the 0.30% P diet. Diets deficient in P often reduce intake (Riddell et al., 1934; Call et al., 1986; Geisert et al., 2010), thus, the 0.10% P diet, likely did not supply adequate P, despite dietary phytase inclusions. In addition, the 0.10% P diet contained corn starch instead of corn to achieve the reduced P. Highly processed ingredients, like corn starch, are known to increase digestibility (Murphy et al., 1994). However, they may also decrease pH. In fact, steers fed the 0.10% P diet had ruminal pH below 5.5 by 6 hours post-feeding, and, thus, were more acidotic (P = 0.01) than steers fed 0.30% P. There were no differences (P > 0.35) in water intake and urine output between steers fed differing concentrations of dietary P. There was no difference (P = 0.86) in g/d intake of N nor was N retention affected (P = 0.84). Nitrogen retention was unaffected because steers fed the 0.10% P diet tended (P = 0.10) to absorb more N and excreted more (P = 0.02) N in the urine and less (P < 0.01) N in the feces compared to steers fed 0.30% P. Concentration of P in the diets fed in this study, affected P digestibility. Steers fed 0.10% P consumed less (P < 0.01) P and excreted less P in feces (P < 0.01) and urine (P < 0.01) than steers fed 0.30% P. In steers consuming the 0.10% P diet, excretion of P was greater than intake of P. Therefore, both retention of P (P < 0.01) and absorption of P (P < 0.01) were negative. Steers fed the 0.10% P diet had an average plasma P concentration of 2.87 mg/dL, suggesting they were, in fact, P-deficient. While steers on the 0.30% P diets, had plasma P concentrations in the P-adequate range, defined as greater than 4.50 mg/dL (NRC, 2000). Water-soluble P concentration in the feces was greater (P < 0.01) in steers fed 0.30% P. However, the proportion of total fecal P that was excreted as water-soluble P was increased by 23.0% in steers fed 0.10% P compared to steers fed 0.30% P, regardless of phytase inclusion level. Not only did steers fed 0.30% P consume more (P < 0.01) total P but they consumed 100% more phytate-P as a proportion of total dietary P than cattle fed 0.10% P. Although there was no interaction between phytase and P concentration in the current study, conclusions can be drawn from metabolism of P in steers fed either P-deficient or P-adequate diets. Steers that consumed a P-deficient diet, 0.10% P, had decreased DMI, which led to decreased fecal output, and these steers were considered to be in negative P balance due to negative absorption and retention values. Diets were formulated to provide similar N per day, and although steers tended to absorb more N during P deficiency, they excreted more N in their urine, thus N retention was not altered. Changes in blood P concentration, as a method to evaluate P status in ruminants, can be determined within 20 d consumption of a P-deficient diet.
Issue Date:2016-12-09
Rights Information:Copyright 2016 Chloe Long
Date Available in IDEALS:2017-03-01
Date Deposited:2016-12

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